Advances in the development of light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) are making these devices brighter, more efficient, and hence increasingly suitable in general area and space illumination applications. These devices are becoming increasingly competitive with incandescent, fluorescent, and high-intensity discharge lamps which have traditionally dominated illumination markets.
The brightness of a LED can be varied by changing the electrical current through the device. However, the amount of light any two LEDs of the same type and make emit as a consequence of the same current can differ noticeably, typically even if for example all other relevant operating conditions are the same. These undesired variations in LED operation can be reduced in several ways. For example, LEDs can be binned into groups of devices with closely matching operating characteristics or LED control circuits can be appropriately calibrated to substantially accurately control the amount of light emitted by the LEDs.
One solution for the above noted problem is to control the direct current through two or more LEDs in a linear control circuit system. A problem however, with using a linear control circuit is that it can dissipate relatively large amounts of energy which would consequently reduce the energy efficiency of the whole system.
Until single LEDs become sufficiently bright, effective area and space lighting systems will typically require many LEDs. The control of lighting devices with many LEDs can be complex. For example, dimming by reducing the drive current in direct current driven LEDs may cause LED flickering. In addition, at low current levels, some LEDs may turn OFF whereas other LEDs with lower forward voltages may remain ON. This can result in the lighting system generating undesired forms of illumination.
For example, U.S. Pat. No. 6,362,578 provides a control method wherein a feedback voltage converter is used to maintain a constant load voltage across a series of strings of LEDs wherein biasing resistors are used for current control. A transistor is connected on the low side of the LEDs and is switched with pulse width modulation (PWM) for brightness control. This configuration enables full dimming control as the current is switched and repetitively turns the LEDs ON and OFF. The time-averaged brightness of the light emitted by the LED is determined by the PWM duty cycle factor. The problem with these types of configurations, however, is that they can be inefficient due to the power losses in the biasing resistor, and may require custom resistors to accurately control the current. More energy efficient solutions may also control the voltage which is supplied to the current controller. For example, a buck-boost regulator can be used to generate a regulated common voltage supply for the high side of an LED array. Low side ballast resistors can then be used to set the LED current, and additional resistors can be used to monitor the LED drive current.
U.S. Pat. No. 4,001,667 also discloses a closed loop circuit that provides constant current pulses, however, this circuit does not allow for full duty cycle control.
U.S. Pat. No. 6,586,890 discloses a method that uses current feedback to adjust power to LEDs by PWM switching the power supply in order to adjust the brightness of the LEDs. A problem with this method is that the disclosed PWM frequency signal is within the range of about 20 Hz to 20 kHz and therefore the power supply is prone to generate audible noise. Depending on the type of LED, low switching frequencies, typically below the low 104 Hz range, can also cause significant thermal cycling and induce thermal stress within the LED which can increase the risk of device failure.
U.S. Pat. No. 6,734,639 discloses a method for controlling overshoots of a switched drive circuit for LED arrays by means of a voltage converter combined with a customized sample and hold circuit. The switching signal controlling the LEDs also controls the enabling and disabling of a voltage converter and thus switches both the load (LEDs) and the supply. The signal controlling the switching of the load current is biased such that it operates the switch essentially in its linear region to be able to control the peak current which can otherwise result in power losses within the switch and reduce the overall system energy efficiency. This configuration however, typically only works for switching frequencies in the range of about 400 Hz and typically does not allow for high frequency switching of the load for example at frequencies above about 20 kHz.
In addition, U.S. Patent Application Publication No. 2004/0036418 discloses a method of driving several strings of LEDs in which a converter is used to vary the current through the LEDs. A current switch is implemented to provide feedback. This method is similar to using a standard buck converter and can provide an efficient way for controlling the current through the LEDs. A problem arises, however, when multiple LED strings require different forward voltages. In this scenario, high-side transistor switches are used as variable resistors to limit the current to a respective LED string. These high side transistor switches can induce losses and decrease the overall energy efficiency of the circuit.
Therefore, there is a need for an adaptive control method and apparatus for a solid-state lighting device that can control voltages supplied to one or more electronic devices.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.